Natural gas production may be complicated by the presence of certain heavy hydrocarbons in the subterranean formation in which the gas is found. Under conditions prevailing in the subterranean reservoirs, the heavy hydrocarbons may be partially dissolved in the compressed gas or finely divided in a liquid phase. The decrease in temperature and pressure attendant to the upward flow of gas as it is produced to the surface results in the separation of solid hydrocarbonaceous material from the gas. Such solid hydrocarbons may form in certain critical places such as on the interior wall of the production string, thus restricting or actually plugging the flow passageway.
Many hydrocarbonaceous mineral streams contain some small proportion of these diamondoid compounds. These high boiling, saturated, three-dimensional polycyclic organics are illustrated by adamantane, diamantane, triamantane and various side chain substituted homologues, particularly the methyl derivatives. Diamondoid compounds have high melting points and high vapor pressures for their molecular weights and have recently been found to cause problems during production and refining of hydrocarbonaceous minerals, particularly natural gas, by condensing out and solidifying, thereby clogging pipes and other pieces of equipment. For a survey of the chemistry of diamondoid compounds, see Fort, Jr., Raymond C., The Chemistry of Diamond Molecules, Marcel Dekker, 1976.
In recent times, new sources of hydrocarbon minerals have been brought into production which, for some unknown reason, have substantially larger concentrations of diamondoid compounds. Whereas in the past, the amount of diamondoid compounds has been too small to cause operational problems such as production cooler plugging, now these compounds represent both a larger problem and a larger opportunity. The presence of diamondoid compounds in natural gas has been found to cause plugging in the process equipment requiring costly maintenance downtime to remove. On the other hand, these very compounds which can deleteriously affect the profitability of natural gas production are themselves valuable products.
Various processes have been developed to prevent the formation of such precipitates or to remove them once they have formed. These include mechanical removal of the deposits and the batchwise or continuous injection of a suitable solvent. Recovery of one such class of heavy hydrocarbons, i.e. diamondoid materials, from natural gas is detailed in commonly assigned co-pending U.S. Pat. Application Ser. No. 405,119, filed Sept. 7, 1989, which is a continuation of U.S. Pat. No. 358,758, filed May 26, 1989, now abandoned, as well as U.S. Pat. Applications Ser. Nos. 358,759; 358,760; and U.S. Pat. Nos. 4,952,747; 4,952,747; 4,952,749; and 4,982,049; 358,761, all filed May 26, 1989. The text of these U.S. Patent Applications is incorporated herein by reference.
Research efforts have more recently been focused on separating diamondoid compounds from the liquid solvent stream described, for example, in the above cited U.S. Pat. Application No. 405,119 U.S. Pat. No. 4,952,748. The diamondoid and solvent components have proven difficult to separate via conventional multistaqe distillation due at least in part to the overlapping boiling ranges of the preferred solvents and the commonly occurring diamondoid compounds. Further, the diamondoid compounds have been found to deposit precipitate in the overhead condenser circuit of a solvent distillation apparatus. Thus researchers have concentrated efforts both on separating the diamondoid compounds from the solvent as well as on utilizing the separated diamondoid materials.
Solvent injection has been found to be essential to produce natural gas economically from wells containing substantial diamondoid concentrations. The resulting diamondoid-enriched solvent may be processed to recover the diamondoid materials if suitable processing equipment is available, and if the market demand for diamondoids provides the economic incentive. However, the geographic location of the diamondoid-producing wells or other factors such as weak market demand, may render diamondoid recovery from the enriched solvent stream uneconomical.
The diamondoid-enriched solvent may be sold by blending the mixture into a heavier fuel such as No. 4 fuel oil or a heavier industrial steam boiler grade fuel oil, such as bunker fuel, assuming that the diamondoid materials could be diluted sufficiently to avoid the deposition of crystalline solids in the fuel oil handling equipment.
The most desirable solvent for recovering diamondoid materials from natural gas would be a light kerosene or diesel fuel. Blending diesel fuel or light kerosene into heavy fuel oil would downgrade the value of the light distillate solvent to that of the heavy fuel oil. Thus, a relatively expensive solvent mixture would be undesirably downgraded and sold as a less valuable fraction. On the other hand, increasingly stringent waste disposal regulations could require that the enriched solvent be handled as a hazardous waste, further restricting the disposition of the enriched solvent.
Thus it would be highly beneficial to provide an integrated process for converting the diamondoid-enriched solvent stream into a readily saleable product. Further, it would be desirable to convert the enriched solvent stream to a more valuable product such as motor gasoline. Still further, it would be desirable to convert the enriched solvent in an integrated process which is compatible with the utilities available at natural gas processing facilities.